1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, using a crystalline thin-film semiconductor and, more particularly, to a method of fabricating planar type thin-film transistors.
2. Description of Related Art
In recent years, techniques for fabricating thin-film transistors (TFTs) on cheap glass substrates have evolved rapidly, because there is an increasing demand for active matrix liquid crystal displays.
An active matrix liquid crystal display has millions of pixels arranged in rows and columns. TFTs are arranged at these pixels. Electric charge going in and out of each electrode at the pixels is controlled by the switching action of the TFTs.
Therefore, if one TFT fails to operate, then pixel electrodes connected with the faulty TFT do not act as display elements. This gives rise to a so-called point defect. For example, in the case of a normally black liquid crystal display, when white color is displayed, the point defect appears as a black point, which is deeply harmful to the appearance. Furthermore, it has been required that a circuit (known as a peripheral driver circuit) for driving TFTs for displaying these pixel electrodes be formed out of TFTs integrated with the former TFTs on the same glass substrate.
In this case, if one driving TFT fails to operate, all TFTs applied with a driving voltage from the faulty TFT do not act as switching elements. This results in a so-called line defect. This is a fatal hindrance to the liquid crystal display.
Accordingly, in an active matrix liquid crystal display, millions of TFTs must operate normally and stably over a long term. However, the present situation is that it is difficult to eliminate point defects and line defects perfectly. One of the causes is poor contact. Poor contact is that an interconnect electrode is poorly electrically connected with an associated TFT at a contact location, thus a defective operation is occurred. Especially, in the case of a planar TFT, poor contact presents serious problems, because an interconnect electrode is electrically connected with an associated TFT through a thin contact hole.
The poor contact is a main cause of premature deterioration of semiconductor device characteristics. Especially, where large currents flow or the device is operated at high temperatures, the deterioration coursed by the poor contact is promoted. Therefore, it is said that the reliability of contacts determines the reliability of the semiconductor device.
Generally, in the case of pixel display regions of an active matrix liquid crystal display, the gate electrodes are brought out of the pixel display regions directly and so no contacts exist. That is, contact with the pixel electrodes is very important for the reliability of the liquid crystal display.
In the case of a peripheral driver circuit, very many (from tens of thousands to millions) contacts exist. Especially, because there exist gate electrode contacts, and because the temperature is elevated by large-current operation, the contacts must have higher reliability than the pixel display regions.